A thermal processing system which heats up a wafer is a common part of the integrated circuit fabrication process. The wafer may require heating to high temperatures such as 200.degree. Celsius to 1500.degree. Celsius for common processing steps such as annealing or reflow. RTP (Rapid Thermal Processing) systems heat up the wafer to a relatively high temperature within a relatively short period of time, and are prevalently used in integrated circuit fabrication systems.
A thermal processing recipe typically includes a step of heating a product wafer to a specified temperature for a specified time period in accordance with a specified temperature versus time characteristic to achieve a specified result. In order to follow the recipe with precision, the wafer temperature achieved for various temperature settings within a thermal processing system may be monitored.
A common way of monitoring the wafer temperature achieved within the thermal processing system heats up a test wafer and then measures the sheet resistance of the test wafer after the test wafer cools down. This measured sheet resistance is correlated to the test wafer temperature that was achieved during heating of the test wafer. The test wafer is pretreated with a specified process that is known to provide an accurate correlation of sheet resistance (measured after the test wafer has been heated up within the thermal processing system) to the temperature that was achieved at the test wafer during heating of the test wafer within the thermal processing system.
Some prior art thermal processing systems are not emissivity independent meaning that a same wafer temperature is not achieved for two wafers having different prior process histories when those two wafers are thermally processed following a substantially same thermal processing recipe. Thus, in prior art thermal processing systems which are not emissivity independent, a test wafer, which typically has a pretreatment different from the product wafer, cannot accurately reflect the temperature reached at the product wafer.
Accordingly, in the prior art nonemissivity-independent thermal processing system, such a test wafer is used in isolation to monitor the calibration of temperatures achieved within the thermal processing system for various temperature settings. The temperature achieved on a product wafer during actual processing of the product wafer is not determined. However, determination of the temperature achieved on the actual product wafer during thermal processing would allow more accurate control during thermal processing of the product wafer.
More recently, thermal processing system manufacturers are providing thermal processing systems which are emissivity independent such as that available from Applied Materials, Inc, Santa Clara, Calif. Two wafers having different prior process histories achieve substantially same temperatures with substantially same thermal processing recipes within an emissivity independent thermal processing system.
With such an emissivity independent thermal processing system, the temperature achieved at an actual product wafer during processing of the product wafer may be determined from measuring the sheet resistance of a test wafer. Such a determination would allow more accurate control of thermal processing of product wafers.